The present invention relates to a communication terminal and more particularly to a communication terminal for automobile used in the Intelligent Transport System (ITS) or the like.
Presently, a study is being made of various wireless services such as ITS. For these services to be versatile requires that highly efficient, inexpensive communication terminals be provided. ITS includes the Electric Toll Collection System (ETC) that uses radio waves of, e.g., 5.8 GHz band to wirelessly collect turnpike tolls, the Road/Automobile Communication System, and the like. ETC and the Road/Automobile Communication System require highly precise communications because the former must correctly collect tolls and the latter must correctly exchange large volumes of information. These terminals are installed with an antenna thereof located inside an automobile or outside it. Locating the antenna outside an automobile requires environmental measures such as dust prevention and rain protection, probably bringing about higher communication terminal costs. On the other hand, in the case where the antenna is located inside the automobile, although the above problem is solved, even if the communication terminals are designed so that antenna impedance matches air impedance, antenna impedance changes depending on installation conditions because matters other than air such as front glass exist in the vicinity of an antenna radiation surface in real environments. As a result, reflection due to impedance mismatch occurs between the antenna and a transmitter-receiver, reducing communication precision. To avoid this, measures are required against loss in the front glass and impedance mismatch between the antenna and transmitter-receiver. The configuration of a communication antenna considering changes in antenna impedance due to front glass for an antenna located inside an automobile is disclosed in, e.g., Japanese Patent Application Laid-Open No. H5-314330 “Noncontact IC Card Mounting Apparatus”. This employs-a method as shown in FIG. 18. That is, a noncontact IC card 204 is attached to front glass 202 by using a mounting apparatus 200, a dielectric member 203 is inserted between the front glass 201 and the antenna 202, and impedance between the antenna 202 and the front glass 201 is matched by the dielectric constant and thickness of the dielectric member 203.
Although the prior art provides impedance matching by inserting a dielectric member for impedance tuning between the antenna and the front glass, this method has a problem in that different dielectric members must be used depending on the thickness and material of the front glass of the automobile. Also, the need to attach the antenna to the front glass limits the freedom to install the communication terminal in any location.